Ultrasonic vibration-assisted machining has several important advantages. One important advantage flows from the friction reducing action, which arises when the cutting edge or point of the tool separates from the work during each vibration cycle and which introduces an ultrasonic pumping action. This pumping action allows transmission of cooling fluids to all areas of the workpieace area. The cooling fluid may be air.
Normal machining has an accuracy of greater than 5 μm and uses ordinary machine tools. Ultrasonic vibration-assisted machining was introduced in the 1960s to provide advantages over normal machining. The expected advantages have not been realized, however, as it did not increase tool life nor improve surface finish. Also, cutting efficiency is low as the cutting speed must be less than the vibration speed.
Since 1980, ultra-precision machining was developed to meet the demands of fabricating complicated optics. A diamond tool bit is the only tool bit that can be used to generate an optical mirror surface finish. However, a diamond tool bit cannot be used on steel due to the strong chemical reaction between the diamond and the steel. The chemical reaction causes graphitization of the diamond.
With prior art systems, the vibration horn is clamped at two static node points of the vibration wave as shown in FIG. 1. The diamond tool bit is mounted on the free end of the vibration horn. To achieve a mirror surface finish, the induced lateral vibration of the horn in the radial direction must be significantly reduced.
For the prior art method of the vibration horn with two static clamping points, there is some distance from the lower static node point to the free end where the tool bit is mounted. In such a case, the point at the free end has reduced stiffness in the lateral/radial direction and lateral vibration in the lateral/radial direction can easily be induced.
To increase the lateral stiffness, the diameter of the horn is increased and, in turn, the power of the ultrasonic vibration generator is also increased. Therefore, the overall size of the apparatus is increased. Furthermore, the operational temperature of the device increases due to the larger power, and is easily damaged due to the higher temperature. Also, lateral (or radial) vibration damages the tool bit. It also causes deeper cuts and therefore lower quality surface finish. Such lateral radial vibrations are normally of the order of 4 μm.
To prevent excessive tool wear, a two stage process has been proposed using electroless nickel plating or coating on a workpiece before machining when fabricating optical mold inserts for the injection molding of plastic lenses. However such methods have many disadvantages. Also, they are not capable of manufacturing moulds with high durability as the nickel tends to lift off the steel workpiece.